| 摘要 |
Optical glasses which are stable to crystallization, of high refractive power, of low colour content, and capable of being fused, which glasses are free from thorium oxide and lie in the optical range defined by the following refractive indices (Nd) and Abb</>ae numbers (vd) .Nd = 1.77.vd = 40 to Nd = 1.81.vd = 36 .Nd = 1.77.vd = 37 to Nd = 1.81.vd = 34 have the composition ..Parts by weight .SiO2 .. .. ...15 -20 .B2O3 .. .. ...8 -10 .BaO .. .. ...27 -37 .CaO + SrO .. ...0 - 3 .Na2O3 .. ...15 24 .ZrO2 .. .. ...3.5 5.5 .TiO2 .. .. ...6 -13 .Ta2O5 or Nb2O5 ...5 - 7 with the proviso that ZrO2 + TiO2 + Ta2O5 or Nb2O5) does not exceed 23 parts by weight CdO and/or PbO and/or Bi2O3 and/or Wo3 0.5-4.5 parts by weight ..Parts by weight .As2O3 .. .. ...0.5-1.5 The alkali metal content is kept smaller than 1 part by weight with Li2O less than 0.5 parts by weight. Up to 3 parts by weight of the said BaO content may be replaced by BeO, SrO, CaO or ZnO and up to 2 parts by weight of the said BaO content may be replaced by MgO. Some of the La2O3 content may be replaced by ThO2 if the radioactivity of the glasses can be tolerated.
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